For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive (vertical) load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile (horizontal) forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as 100 feet can be attained in members as deep as three feet for roof loads. The basic principal is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principal, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Prestressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of byproducts from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
Several U.S. patents have considered the problem of anchor and tendon corrosion. U.S. Pat. No. 4,348,844, issued to Morris Schupack et al., on Sept. 14, 1982, disclosed a tendon assembly in which a tendon is enclosed in a sheath suspended under tension between two spaced anchor members. The anchor members are entirely enclosed within an envelope or a housing. The sheath, the envelope, and the housing are required to comprise electrically non-conductive materials for electrically isolating the tendon and anchor members from a surrounding concrete structure to thereby prevent the effects of electrolysis caused by electrical currents.
After experimentation and study, it has been found that electrolytic actions, described in detail in the Schupack patent, have little or no deteriorating effect on the anchor assembly. There are occasions in which the electrolytic action created by currents passing through the tendon to the anchor assembly has been found to be beneficial. For instance, when anodic material is placed in electrical connection with such electrically conductive anchor assembly. In this situation, the anodic material to the structural strength and stability of the cathodic anchor assembly.
U.S. Pat. No. 4,616,458, issued to Davis et al., on Oct. 14, 1986, provided a plastic structure for protecting the anchor assembly and the ends of a tendon from exposure to the corrosive elements. The system of this patent describes a protective top member and a protective bottom member. The anchor was interposed between these members, the members were snap-fitted together, and the anchor locked into position between these protective members. Grease was then injected into the interior between these protective plastic members so as to seal the anchor from the corrosive water in the environment. A grease cap would be threaded onto the protective top member so as to allow grease to be injected into the interior space.
In practice, the device of the Davis patent required extensive manipulation of the top and bottom members so as to allow the snap-fit to occur. It also required the difficult manipulation of fitting the anchor within this assembly. Finally, the step of injecting grease into the interior was required following assembly. It was found that many man-hours were consumed in the assembly and manipulation steps. On occasion, assembly procedures allowed grease to leak from the interior between the top and bottom members.
It is an object of the present invention to provide a tendon tensioning anchor that effectively seals the exterior of the anchor from the exterior environment.
It is another object of the present invention to provide a tendon tensioning anchor that allows electrolytic effects to occur between the tendon and the interior of the anchor.
It is a further object of the present invention to provide a tendon tensioning anchor that has a protective covering which needs no assembly, manipulation, or effects a waste of manpower.
It is still a further object of the present invention to provide a tendon tensioning anchor that effects a superior seal between the exterior of the anchor and the exterior environment.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.